There have been made various proposals on chromogenic dyes which are colorless in themselves but form dyes absorbing electromagnetic waves ranging from the long wavelength region of visible light to the near-infrared region on contact with acidic substances. Among others, phthalide type compounds are known as the compounds having such properties as mentioned above, from Japanese Patent L-O-P Publication Nos. 121035/1976, 121037/1976, and 115456/1980 (U.S. Pat. No. 4,020,056), 115449/1980, 115450/1980, 115451/1980, and 115452/1980 (U.S. Pat. No. 4,022,771), 121038/1976 (U.S. Pat. No. 4,026,883) 167979/1982, 157779/1983, 8364/1985 and 27589/1985. Thiofluoran type compounds are also known from Japanese Patent L-O-P Publication No. 148695/1984, and fluorene type compounds are known from Japanese Patent L-O-P Publication No. 199757/1984 and European Patent Laid-Open-to-Public Publication No. 124377. Furthermore, fluoran compounds have been proposed in Japanese Patent L-O-P Publn. No. 284485/1986.
Most of these chromogenic dyes as proposed above, however, form gray-bluish or gray-greenish colors, and such chromogenic dyes as forming blackish colors and having absorption of electromagnetic waves also in the near-infrared region are not known as yet.
Recently, bar codes have come to be extensively used as a means for controlling distribution of commodities.
In reading markings of the bar codes, there is an increasing tendency that because of their cheapness, reading equipments relying on the use of semiconductor laser have been often used. The semiconductor laser emits visible light rays and near-infrared rays of the wavelength of 650 nm or more, and hence the markings which are read out by such equipments should be those which absorb visible light rays to near-infrared rays of the above-mentioned wavelength range. At the same time, in order to confirm that color development has been perfectly effected, it is desirable that in the case of in store marking, the developed color is of blackish hue which can be most clearly confirmed.
Heretofore, numerous fluoran compounds which produce black color have been known, but all the colors produced thereby do not have absorption capacity in the near-infrared wavelength region.
Under such circumstances, there is a pressing need for chromogenic dyes which produce blackish colors and absorb electromagnetic waves not only in the visible light regions but also in the near-infrared regions upon contact with the developers.